Semiconductor devices play a significant role in solving the energy challenges. Specifically, nitride power transistors have great potential in the application of advanced transportation systems, more robust energy delivery networks and many new revolutionary approaches to high-efficiency electricity generation and conversion. Those systems rely on very efficient converters to step-up or step-down electric voltages. Nowadays, these devices are mainly made of silicon (Si). However, the limited breakdown voltage and frequency response of Si, and its higher resistance make the commercial devices and circuits currently available very bulky, heavy and inappropriate for future power applications. As an alternative, gallium nitride (GaN) devices have achieved record combination of high-voltages, high frequency response and low on-resistances for power applications.
Currently, GaN power devices, such as the GaN-based high electron mobility transistors (HEMTs), are regarded as one of the most promising candidates for high-power, high-voltage and high frequency applications. GaN HEMTs have achieved up to 10 times higher power density of GaAs HEMTs with much larger breakdown voltage (VB) and current density, as well as a high cut-off frequency of over 400 GHz. State-of-the-art power levels have been demonstrated on SiC substrates with total output powers of 800 W at 2.9 GHz and over 500 W at 3.5 GHz. However, for the high-power applications, such as high-power motors, a higher output power, i.e. 3-5 kW, is desired, which requires a further enhancement of output power of GaN power devices.
To achieve the high-power requirement for GaN power devices, the device current capability needs to be enhanced. For instance, in the GaN power devices used for amplifiers, such as the class A devices, the maximum output power can be derived as
      P    out    max    =            1      8        ⁢                  I        max            ⁡              (                              V                          m              ,              max                                -                      V            k                          )              ⁢                  ⁢    where    ⁢                  ⁢          V              m        ,        max            is determined by the device breakdown voltage, Vk is the knee voltage and Imax is the maximum drain-source current. For GaN-based power devices, although the device breakdown voltage can increase with increasing gate-to-drain distance, the Imax is currently limited by the transport capability of the single channel, where I saturates at high electric field due to the saturation of carrier mobility and velocity and the alleviated channel temperature.
The multichannel structures of GaAs-based and GaN-based HEMTs can be used to increase the device current density. In AlGaAs/GaAs HEMTs, the utilization of up to three channels with 2-3×1012 cm−2 electrons in each could achieve a high output current as ˜1 A/mm. In AlGaN/GaN HEMTs, double-channel structures can not only enable a high output current, but also reduce the differential source access resistance for higher linearity and gain cutoff frequency.
However, the double-channel structure of HEMTs, although delivered a higher drain current density, can have large subthreshold swing, low threshold voltage, short-channel effects and non-linear gate transconductance, due to a weak gate control over the lower 2DEG channel. The weak gate control not only harms the benefits of double-channel device in current enhancement, but inhibits the development of multi-channel devices, as it is more difficult for gate to turn on or off more channels below the first channel.
One possible solution to the weak gate problem is to use a back-barrier to enhance the electron confinement for the dipper channels and the gate control. However, the transfer characteristics of the double-channel HEMTs with back-barrier are still much inferior to that of single-channel HEMTs.
Another solution provides spaced apart gate structures deeply etched to access the dipper channels. However, this solution sacrifices the gate width, making only ˜½ gate width actually usable, which greatly reduced the effective current density (total current over total device width) and therefore loses the advantage of current enhancement brought by multi-channels. In some implementations, the current density of this exemplar multi-channel HEMTs can be even lower than the current density of single-channel devices. On the other hand, as current needs to aggregate into the regions between the spaced apart gates, current crowding and electric field crowding near the gate is inevitable, which would typically lead to high channel temperature, preliminary device breakdown and poor device reliability.